1. Field of the Invention
This invention relates to a resin-cemented optical element, a mold used for producing the element, an optical article (or device) having the element, and fabrication process of the element
2. Description of the Related Art
At present, optical elements are used in various fields. Depending on the purpose for which they are used, it is difficult to materialize required optical characteristics and so forth in some cases in respect of conventional spherical lenses. Accordingly, aspheric lenses are attracting notice. “Aspheric lens” is a generic term for lenses the curvature of which is kept continuously different over the region extending from the lens center toward the periphery. The use of aspheric lenses at some part of optical systems enables considerable reduction of the number of lenses necessary for the correction of aberrations, compared with a case where the optical system is comprised of only spheric lenses. This enables downsizing and weight reduction of the optical system. Also, the use of aspheric lenses enables high-grade correction of aberrations which is difficult for spherical lenses, and hence can bring about an improvement in image quality.
Aspheric lenses having such superior characteristics have not necessarily come into wide use. The greatest reason therefor can be said to be a difficulty in working. Conventional aspheric lenses have only be able to be produced by precisely polishing base members made of glass, and have involved the problem of a high processing cost.
In recent years, however, resin-cemented optical elements that materialize aspherical shapes by the aid of resin layers which can be made into any desired shapes with much greater ease than the precise polishing of glass have been put into practical use, so that aspheric lenses have rapidly come into wide use.
The resin-cemented optical element is an element in which a resin layer has been cemented to the surface of a base member made of glass or the like. This resin-cemented optical element is produced by a process such as a composite-type aspherical-surface molding process, in which, using a mold (such as a metal mold), a resin composition (inclusive of a resin precursor composition) is poured into a space between a base member and the mold, followed by curing to form on the base member surface a resin layer having any desired shape. In the present specification, a lens produced by this composite-type aspherical-surface molding process may be called a PAG (plastic adhesion glass) lens.
In the case when the resin-cemented optical element is thus produced by the composite-type aspherical-surface molding process, the base member may break when the resin cured on the base member is released from the mold. This phenomenon is remarkable especially when the resin layer has a large thickness. Accordingly, it has been impossible in practice to produce any PAG lens having a thick resin layer of 850 μm or larger in maximum layer thickness.
This phenomenon is considered to be caused by the adhesion of the resin layer to the mold. Usually, the resin is released from the mold by means of an ejector (ejection member) in such a way that a force acting in the direction where the former is released from the latter is applied to the base member at its part standing uncovered to the periphery of the element. Here, in the event that the resin layer remains without being released, in the state it has been kept adhered to the mold until the amount of deformation of the base member exceeds a tolerance limit, the base member breaks because of the distortion due to a deformation having exceeded the tolerance limit.